Gas diffusion electrode substrate and method for producing gas diffusion electrode substrate

ABSTRACT

The purpose of the present invention is to provide: a method for producing a gas diffusion electrode base which enables the achievement of a gas diffusion electrode base that has a microporous layer with small surface roughness and is not susceptible to damaging an electrolyte membrane; and a gas diffusion electrode base that has a microporous layer with small surface roughness and is not susceptible to damaging an electrolyte membrane. For the purpose of achieving the above-described purpose, the present invention has the configuration described below. Namely, a specific gas diffusion electrode base which has a carbon sheet and a microporous layer, and wherein the carbon sheet is porous and the DBP oil absorption of a carbon powder contained in the microporous layer is 70-155 ml/100 g.

TECHNICAL FIELD

The present invention relates to a gas diffusion electrode substratewhich is suitably used for a fuel cell, particularly for a polymerelectrolyte fuel cell and in which a microporous layer is formed on asurface of a carbon sheet; and a method for producing a gas diffusionelectrode substrate.

BACKGROUND ART

A polymer electrolyte fuel cell in which a hydrogen-containing fuel gasand an oxygen-containing oxidizing gas are supplied to an anode and acathode, respectively, and an electromotive force is generated by anelectrochemical reaction occurring at both poles is generallyconstituted by sequentially stacking a bipolar plate, a gas diffusionelectrode substrate, a catalyst layer, an electrolyte membrane, acatalyst layer, a gas diffusion electrode substrate, and a bipolarplate. The gas diffusion electrode substrate is required to have highgas diffusivity for allowing a gas supplied from the bipolar plate to bediffused into the catalyst layer and high water removal performance fordischarging water generated by the electrochemical reaction to thebipolar plate, as well as high electrical conductivity for extractinggenerated electric current, and a gas diffusion electrode substrate iswidely used in which a microporous layer is formed on a surface of asubstrate, or a carbon sheet composed of a carbon fiber and the like.

One of known problems of such a gas diffusion electrode substrate,however,is that the surface roughness of the microporous layer is largeto damage an electrolyte membrane in the polymer electrolyte fuel cell,deteriorating the durability of the electrolyte membrane, and manyapproaches have been made to solve this problem.

For example, Patent Document 1 proposes a microporous, layer including acarbon powder small in particle size.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: WO 2005/081339 A1

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In the gas diffusion electrode substrate described in Patent Document 1,the particle size of the carbon powder included in the microporous layeris decreased to reduce the surface roughness of the microporous layer,so that the microporous layer is supposedly less likely to damage anelectrolyte membrane to improve the durability of the electrolytemembrane. Patent Document 1 also supposes that a smoother surface, i.e.,a surface small in surface roughness can be obtained as the particlesize of the carbon powder decreases. The carbon powder small in particlesize, however, have not sufficiently reduced the surface roughness ofthe microporous layer.

The present inventors have considered that when the particle size of thecarbon powder included in the microporous layer is small, a coatingliquid for forming the microporous layer (hereinafter, also described asan MPL coating liquid) infiltrates into a carbon sheet in an applicationstep of applying the MPL coating liquid onto the carbon sheet(hereinafter, also described as a MPL application step), so that thesurface roughness of the microporous layer is not sufficiently reduced.The present inventors have also noticed a problem that the surfaceroughness is not sufficiently reduced in some cases even when theparticle size of the carbon powder is decreased.

An object of the present invention is to provide, in view of thebackground of such a conventional technique, a method for producing agas diffusion electrode substrate, the method being capable of giving agas diffusion electrode substrate whose microporous layer, is small insurface roughness to be less likely to damage an electrolyte membrane;and a gas diffusion electrode substrate whose microporous layer is smallin surface roughness to be less likely to damage an electrolytemembrane.

Solution to the Problem

The present inventors have conducted earnest studies repetitively tosolve the above problem and focused attention on the secondary particlesize of the carbon powder, not the primary particle size of the carbonpowder, to reduce the surface roughness of the microporous layer. Thepresent inventors have found that the surface roughness of themicroporous layer can be sufficiently reduced by using a carbon powderhaving a dibutyl phthalate oil absorption (DBP oil absorption), as anindex for the secondary particle size, in an appropriate range.

The gas diffusion electrode substrate and the method for producing a gasdiffusion electrode substrate according to the present invention includethe following configuration.

(1) A gas diffusion electrode substrate including a carbon sheet and amicroporous layer, wherein

the carbon sheet is porous,

a carbon powder included in the microporous layer has a DBP oilabsorption of 70 to 155 ml/100 g,

the microporous layer has an infiltration index (L/W) of 1.10 to 8.00,the infiltration index being calculated from the areal weight (W) of themicroporous layer and the thickness (L) of the microporous layer, and

the microporous layer has a thickness (L) of 10 to 100 μm.

(2) A gas diffusion electrode substrate including a carbon sheet and amicroporous layer, wherein

the carbon sheet is porous,

a carbon powder included in the microporous layer has a DBP oilabsorption of 70 to 155 ml/100 g, and

the microporous layer has a surface roughness of 3.0 to 7.0 μm.

(3) A gas diffusion electrode substrate including a carbon sheet and amicroporous layer, wherein

the carbon sheet is porous,

a carbon powder included in the microporous layer has a DBP oilabsorption of 70 to 155 ml/100 g, and

the gas diffusion electrode substrate has a variety in thickness of 10.0μm or less.

(4) A method for producing a gas diffusion electrode substrate includinga carbon sheet and a microporous layer, wherein

the carbon sheet is porous,

the method includes an application step of applying a coating liquid forforming the microporous layer (hereinafter, described as an MPL coatingliquid) onto at least one surface of the carbon sheet by a slit diecoater (hereinafter, described as an MPL application step),

the slit die coater has a lip-tip length of 0.10 to 10.00 mm, and

the MPL coating liquid contains a carbon powder having a DBP oilabsorption of 70 to 155 ml/100 g and an ash content of less than 0.10%by mass, and contains a dispersion medium.

Effects of the Invention

The present invention is a method for producing a gas diffusionelectrode substrate, the method being capable of giving a gas diffusionelectrode substrate including a microporous layer small in surfaceroughness and an electrolyte membrane high in durability; and a gasdiffusion electrode substrate including a microporous layer small insurface roughness and an electrolyte membrane high in durability.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic sectional view that illustrates one aspect of aslit die coater used for a method for producing a gas diffusionelectrode substrate according to the present invention.

MODE FOR CARRYING OUT THE INVENTION

A gas diffusion electrode substrate of the present invention includes acarbon sheet and a microporous layer. In more detail, the gas diffusionelectrode substrate of the present invention has the microporous layerformed on at least one surface of the carbon sheet.

First described is the carbon sheet as a constituent of the presentinvention. It is important that the carbon sheet of the presentinvention is porous. A porous carbon sheet can achieve both excellentgas diffusivity and excellent water removal performance. For making thecarbon sheet porous, porous material is preferably used as a materialused for preparing the carbon sheet.

The carbon sheet in the present invention is required, to have high gasdiffusivity in an in-plane direction and in a through-plane directionfor allowing a gas supplied from a bipolar plate to be diffused into acatalyst and high water removal performance for discharging watergenerated by an electrochemical reaction to the bipolar plate, as wellas high electrical conductivity for extracting generated electriccurrent.

Therefore, preferably used as the carbon sheet is a carbonfiber-containing porous material such as carbon fiber woven fabric,carbon fiber non-woven fabric, or a carbon fiber papermaking substratebecause of its excellent corrosion resistance, and especially, a carbonsheet having a carbon fiber papermaking substrate bonded with carbide,namely “carbon paper,” is preferably used because of its excellentmechanical strength. Additionally, “carbon cloth” as the carbon fiberwoven fabric or felt-type carbon non-woven fabric as the carbon fibernon-woven fabric can also be used as the carbon sheet.

Hereinafter, a case of using the carbon fiber papermaking substrate isdescribed as a representative example.

In the present invention, a carbon sheet having a carbon fiberpapermaking substrate bonded with carbide can normally be obtained byimpregnating a carbon fiber papermaking substrate with a resin andcarbonizing the resulting substrate, as described below.

Examples of the carbon fiber include polyacrylonitrile (PAN)-based,pitch-based and rayon-based carbon fibers. Especially, a PAN-based orpitch-based carbon fiber is preferably used in the present inventionbecause of its excellent mechanical strength.

The carbon fiber in the present invention has a mean diameter ofmonofilaments preferably in the range of 3 to 20 and more preferably inthe range of 5 to 10 μm. A mean diameter of 3 μm or more, morepreferably 5 μm increases the pore size of the carbon sheet to improvethe water removal performance, so that flooding can be suppressed.Meanwhile, the mean diameter is preferably 20 μm or less and morepreferably 10 μm. The use of a carbon sheet having such a mean diameterdecreases the diffusivity of water vapor to improve cell performance ata high temperature. The flooding refers to a phenomenon that the carbonsheet is clogged with a large amount of water generated when a fuelcell, particularly a polymer electrolyte fuel cell is operated in a highcurrent density region. When such a phenomenon occurs during theoperation of a fuel cell, the supply of a fuel gas becomes insufficientto cause a problem of deteriorating the cell performance.

Here, the mean diameter of monofilaments in the carbon fiber isdetermined by taking a photograph of carbon fibers at a magnification of1000 times under a microscope such as a scanning electron microscope,randomly selecting 30 different monofilaments, measuring theirdiameters, and deriving the average value of their diameters. As thescanning electron microscope, S-4800 manufactured by Hitachi, Ltd. orits equivalent product can be used.

The carbon fiber in the present invention has a mean length ofmonofilaments preferably in the range of 3 to 20 mm and more preferablyin the range of 5 to 15 mm. A mean length of 3 mm or more, morepreferably 5 mm or more preferably makes the carbon sheet excellent inmechanical strength, electrical conductivity, and thermal conductivity.Meanwhile, a mean length of 20 mm or less, more preferably 15 mm or lessmakes the dispersibility of carbon fibers during paper making excellentto preferably give a uniform carbon sheet. A carbon fiber having such amean length can be obtained by a method of cutting a continuous carbonfiber into a desired length, or the like.

Here, the mean length of the carbon fiber is determined by taking aphotograph of carbon fibers at a magnification of 50 times under amicroscope such as a scanning electron microscope, randomly selecting 30different monofilaments, measuring their lengths, and deriving theaverage value of their lengths. As the scanning electron microscope,S-4800 manufactured by Hitachi, Ltd. or its equivalent product can beused. Although the mean diameter and the mean length of monofilaments inthe carbon fiber are normally measured by directly observing a carbonfiber as a raw material, it may also be measured by observing the carbonsheet.

In the present invention, the carbon sheet is preferably low in arealweight, having an areal weight in the range of 20 to 75 g/m². The carbonsheet has an areal weight of preferably 70 g/m² or less and morepreferably 65 g/m² or less. The carbon sheet also has an areal weight ofpreferably 25 g/m² or more and more preferably 30 g/m² or more. A carbonsheet having an areal weight of 20 g/m² or more, more preferably 25 g/m²or more, further preferably 30 g/m² or more is further improved inelectrical conductivity to more increase the electrical conductivity ofthe gas diffusion electrode substrate including the carbon sheet, sothat the cell performance is further improved at both high and lowtemperatures. Further, the mechanical strength of the carbon sheet canbe further improved to more preferably support an electrolyte membraneand a catalyst layer. Meanwhile, a carbon sheet having an areal weightof 75 g/m² or less, more preferably 70 g/m² or less, further preferably65 g/m² or less increases the gas diffusivity in the through-planedirection of a resultant gas diffusion electrode substrate to furtherimprove the cell performance at both high and low temperatures.

The carbon sheet having such an areal weight can be obtained bycontrolling the areal weight of the carbon fiber in a prepreg and theaddition amount of a resin component based on the carbon fiber, in theproduction method described below. In the present invention, describedas the “prepreg” is one obtained by impregnating, for example, a carbonfiber-containing papermaking substrate with a resin composition to be abinder material. In the present invention, the binder materialrepresents a component other than the carbon fiber in the carbon sheet.The binder material includes a resin composition and/or carbide thereofas a material that acts for bonding carbon fibers. When a hydrophobicmaterial is used for the carbon sheet, the hydrophobic material isincluded in the binder material. In the present invention, the “resincomposition” represents, for convenience, a resin composition, a resincomposition and carbide thereof, or carbide of a resin composition. Thecarbide of a resin composition is one obtained by carbonizing a resincomponent in the resin composition. Here, a carbon sheet having a lowareal weight can be obtained by decreasing the areal weight of thecarbon fiber in a prepreg, and a carbon sheet having a high areal weightcan be obtained by increasing the areal weight of the carbon fiber. Acarbon sheet having a low areal weight can also be obtained bydecreasing the addition amount of a resin component based on the carbonfiber, and a carbon sheet having a high areal weight can also beobtained by increasing the addition amount of a resin component. In thepresent invention, the areal weight means the mass per unit area.

Here, the areal weight of the carbon sheet can be obtained by dividingthe mass of the carbon sheet weighed using an electronic balance by thearea of the carbon sheet.

In the present invention, the upper limit of the thickness of the carbonsheet is preferably 200 μm, more preferably 160 μm, and furtherpreferably 140 μm. The lower limit of the thickness of the carbon sheetis preferably 50 μm, more preferably 60 μm, and further preferably 70μm. A carbon sheet having a thickness of 50 μm or more, more preferably60 μm or more, further preferably 70 μm or more makes the gas diffusionin the in-plane direction more preferable and further facilitates thesupply of a gas to a catalyst under ribs of a bipolar plate, so that thecell performance is further improved at both high and low temperatures.Further, the mechanical strength of the carbon sheet can be furtherimproved to more preferably support an electrolyte membrane and acatalyst layer. Meanwhile, a carbon sheet having a thickness of 200 gmor less, more preferably 160 μm or less, further preferably 140 μm orless allows the water removal path to be shortened to further improvethe water removal performance, more suppressing the flooding, and alsoallows the electrical conductive path to be shortened to further improvethe electrical conductivity, more improving the, cell performance atboth high and low temperatures.

The carbon sheet having such a thickness can be obtained by controllingthe thickness during annealing, in the production method describedbelow. Here, the thickness of, the carbon sheet can be determined usinga micrometer in a state of compressing the carbon sheet at a surfacepressure of 0.15 MPa.

In the gas diffusion electrode substrate of the present invention, it ispreferred that one surface of the carbon sheet have a covering rate of70 to 90%, the other surface have a covering rate lower than thecovering rate of the one surface by 5 to 20 points, and the microporouslayer be on a side of the one surface of the carbon sheet. Here, thephrase “the other surface has a covering rate lower than the coveringrate of the one surface by 5 to 20 points” means that the difference inthe covering rate determined between one surface having a largercovering rate and the other surface having a smaller covering rate is 5to 20 points. In the present invention, the one surface of the carbonsheet (surface having a larger covering rate) preferably has themicroporous layer thereon. The gas diffusion electrode substrateincluding the microporous layer on the one surface (surface having alarger covering rate) of the carbon sheet can be obtained by a method ofapplying an MPL coating liquid onto the one surface (surface having alarger covering rate) of the carbon sheet.

Here, the covering rate is represented by the proportion of a portionwhose surface is covered with the carbon fiber and the binder material(portion where the carbon fiber and the binder material are present), tothe whole surface (whole combined surface of an empty portion and aportion where the carbon fiber and the binder material are present) ofthe carbon sheet. The covering rate can be determined by numericallyprocessing an image obtained by observing the surface of the carbonsheet under a scanning electron microscope. That is, the covering ratecan be obtained by separating the empty portion on the surface from theportion where the carbon fiber and the binder material are present anddetermining the area ratio between the portions.

The covering rate is measured in the following procedure.

First, a surface of the carbon sheet is imaged at a magnification of 50times under a scanning electron microscope (S4800 manufactured byHitachi, Ltd.) while the light-dark contrast is adjusted by anaccompanying automated adjusting function. Next, using the imageprocessing program “J-trim,” the obtained image is sectioned, bybrightness, into 256 grades between the maximum and minimum lightness,and binarized with the 70th grade from the minimum as a threshold. Theproportion of an area binarized to the light side to the whole area isdefined as the covering rate [%]. It is possible to separate the carbonsheet from the gas diffusion electrode substrate and measure thecovering rate of the carbon sheet. For example, the gas diffusionelectrode substrate is heated the atmosphere at 600° C. for 30 minutes,a fluororesin included in the microporous layer of the gas diffusionelectrode substrate is oxidatively decomposed, and then, a ultrasonictreatment is carried out in ethanol, whereby it is possible to take outthe carbon sheet.

In the present invention, the other surface of the carbon sheet has asmaller covering rate than the covering rate of the one surface of thecarbon sheet to allow liquid water in the carbon sheet to move from aside of the one surface having a larger covering rate to a side of theother surface having a smaller covering rate, so that liquid water canbe efficiently discharged from the carbon sheet to a bipolar plate. Sucha configuration not only improves the water removal performance but alsothe gas diffusivity because the inside of the carbon sheet is neverclogged with water. Therefore, the cell performance is improved in ahigh current density region that causes a large amount of water.

Accordingly, there is desirably a certain difference in the coveringrate between both surfaces of the carbon sheet, and the difference inthe covering rate between both surfaces of the carbon sheet ispreferably 5 points or more. Meanwhile, an excessively large differencein the covering rate between both surfaces of the carbon sheet is likelyto cause insufficient mechanical strength of the carbon sheet.

Therefore, the difference in the covering rate is preferably 20 pointsor less. Further, in consideration of the balance between efficientwater removal performance and efficient gas diffusivity, the differencein the covering rate is more preferably 6.5 points or more and 15.0points or less and further preferably 7.5 points or more and 12.0 pointsor less.

A carbon sheet having a covering rate of 70% or more on a surface onwhich the microporous layer is provided makes the MPL coating liquidless likely to infiltrate into the carbon sheet in an MPL applicationstep described below, reducing the surface roughness of the microporouslayer. Meanwhile, a carbon sheet having a covering rate of 90% or lesson a surface on which the microporous layer is provided further improvesthe gas diffusivity in the through-plane direction of the carbon sheetto increase the gas diffusivity in the through-plane direction of thegas diffusion electrode substrate, further improving the cellperformance at both high and low temperatures. Further, in considerationof the balance between efficient water removal performance and efficientgas diffusivity, the one surface of the carbon sheet preferably has acovering rate of 75.0% or more and 81.4% or less.

The gas diffusion electrode substrate of the present invention includesthe carbon sheet and the microporous layer. That is, in the gasdiffusion electrode substrate of the present invention, the microporouslayer is provided on at least one surface of the carbon sheet. Themicroporous layer is required to have high gas diffusivity in thethrough-plane direction for allowing a gas supplied from a bipolar plateto be diffused into a catalyst and high water removal performance fordischarging water generated by an electrochemical reaction to thebipolar plate, as well as high electrical conductivity for extractinggenerated electric current. Furthermore, the microporous layer also hasa function of promoting back-diffusion of moisture to an electrolytemembrane to wet the electrolyte membrane.

In the present invention, the microporous layer includes a carbon powderand is porous from the viewpoint of improving the electricalconductivity and the water removal performance. In order to improve theelectrical conductivity and the water removal performance, a linearcarbon and a fluororesin can be used as the carbon powder and thehydrophobic material, respectively. In the present invention, themicroporous layer more preferably includes a hydrophobic material.

In the present invention, the microporous layer preferably has an arealweight in the range of 10 to 35 g/m². The microporous layer has an arealweight of more preferably 30 g/m² or less and further preferably 25 g/m²or less. The microporous layer also has an areal weight of morepreferably 12 g/m² or more and further preferably 14 g/m² or more. Amicroporous layer having an areal weight of 10 g/m² or more, morepreferably 12 g/m² or more, further preferably 14 g/m² or more can morecover the surface of the carbon sheet to more promote back-diffusion ofgenerated water, so that dry-out of an electrolyte membrane can besuppressed. In addition, a microporous layer having an areal weight of35 g/m² or less, more preferably 30 g/m² or less, further preferably 25g/m² or less improves the water removal performance, so that theflooding can be more suppressed.

In the present invention, examples of the carbon powder contained in themicroporous layer or the MPL coating liquid include carbon powders ofcarbon black such as furnace black, acetylene black, lamp black, andthermal black; graphite such as flaky graphite, scaly graphite, earthygraphite, artificial graphite, expanded graphite, and flake graphite;and CNT. Especially, carbon black is more preferably used, and acetyleneblack is most preferably used because of its less impurities.

In the present invention, when the carbon powder included in themicroporous layer or the MPL coating liquid is a linear carbon, thelinear carbon preferably has an aspect ratio of 30 to 5000. A linearcarbon having an aspect ratio in the above range appropriatelysuppresses the infiltration of a filler-containing coating liquid, or aprecursor of the microporous layer, into the carbon sheet to improve thegas diffusivity in the in-plane direction and the water removalperformance, so that the flooding can be suppressed. Furthermore, themicroporous layer is formed on a front layer of the carbon sheet so asto have a sufficient thickness, to promote back-diffusion of generatedwater, so that dry-out of an electrolyte membrane can be suppressed.

In the present invention, the microporous layer preferably includes ahydrophobic material from the viewpoint of promoting the removal ofliquid water. Especially, a fluorinated polymer is preferably used asthe hydrophobic material because of its excellent corrosion resistance.Examples of the fluorinated polymer include polytetrafluoroethylene(PTFE), a tetrafluoroethylene-hexa fluoro propylene copolymer (FEP), anda tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA).Especially, FEP is more preferable whose viscosity in a molten state islower than the viscosity of PTFE and which has thus less deviation of afluororesin.

In the present invention, the gas diffusion electrode substratepreferably has a thickness in the range of 70 to 230 gm. The gasdiffusion electrode substrate has a thickness of more preferably 190 μmor less and further preferably 170 μm or less. The gas diffusionelectrode substrate also has a thickness of preferably 70 μm or more,more preferably 80 μm or more, and further preferably 90 μm or more. Agas, diffusion electrode substrate having a thickness of 70 μm or more,more preferably 80 μm or more, further preferably 90 μm or more improvesthe gas diffusivity in the in-plane direction and further facilitatesthe supply of a gas to a catalyst under ribs of a bipolar plate, so thatthe cell performance is further improved at both high and lowtemperatures. Meanwhile, a gas diffusion electrode substrate having athickness of 230 μm or less, more preferably 190 μm or less, furtherpreferably 170 μm or less further improves the water removal performanceto more suppress the flooding, and allows the electrical conductive pathto be more shortened to further improve the electrical conductivity,more improving the cell performance at both high and low temperatures.The gas diffusion electrode substrate having such a thickness can beobtained by controlling the thickness of the carbon sheet and thethickness of the microporous layer.

Here, the thickness of the gas diffusion electrode substrate can bedetermined using a micrometer in a state of compressing the gasdiffusion electrode substrate at a surface pressure of 0.15 MPa.

In one aspect of the gas diffusion electrode substrate of the presentinvention, the infiltration index (L/W) is in the range of 1.10 to 8.00that is calculated from the areal weight (W) of the microporous layerand the thickness (L) of the microporous layer, and the thickness (L) ofthe microporous layer is in the range of 10 to 100 μm. The infiltrationindex is more preferably 1.20 or more and further preferably 1.40 ormore. The infiltration index is also more preferably 7.00 or less andfurther preferably 6.00 or less. When the areal weight (W) of themicroporous layer is small and the thickness (L) of the microporouslayer is large, that is, when the infiltration of the MPL coating liquidinto the carbon sheet is small, the infiltration index (L/W) increases.An infiltration index (L/W) of 1.10 or more, more preferably 1.20 ormore, further preferably 1.40 or more gives a gas diffusion electrodesubstrate small in infiltration of the MPL coating liquid into thecarbon sheet, high in gas diffusivity, and high in cell performance. Aninfiltration index (L/W) of 8.00 or less, more preferably 7.00 or less,further preferably 6.00 or less gives the microporous layer a densestructure to reduce the surface roughness of the gas diffusion electrodesubstrate, so that an electrolyte membrane is less likely to be damaged,improving the durability of a fuel cell. The thickness (L) of themicroporous layer is more preferably 15 μm or more and furtherpreferably 20 μm or more. The thickness (L) of the microporous layer isalso more preferably 90 μm or less and further preferably 80 μm or less.A microporous layer having a thickness (L) of 10 μm or more, morepreferably 15 μm or more, further preferably 20 μm or more makes thecarbon sheet less likely to damage an electrolyte membrane, so that thedurability of a fuel cell is improved. A microporous layer having athickness (L) of 100 μm or less more preferably 90 μm or less, furtherpreferably 80 μm or less increases the gas diffusivity of the gasdiffusion electrode substrate to improve the cell performance. The gasdiffusion electrode substrate having an infiltration index and athickness as described above can be obtained by the production methoddescribed below.

In one aspect of the gas diffusion electrode substrate of the presentinvention, the microporous layer preferably has a surface roughness of3.0 to 7.0 μm. More preferable upper and lower limits and the like ofthe surface roughness of the naicroporous layer in the present aspectare similar to more preferable upper and lower limits and the like inanother aspect described below.

In one aspect of the gas diffusion electrode substrate of the presentinvention, the gas diffusion electrode substrate preferably has avariety in thickness of 10.0 μm or less. A more preferable upper limitand the like of the variety in thickness of the gas diffusion electrodesubstrate in the present aspect are similar to a more preferable upperlimit and the like in still another aspect described below.

In the one other aspect of the gas diffusion electrode substrate of thepresent invention, the microporous layer has a surface roughness of 3.0to 7.0 μm. The microporous layer has a surface roughness of morepreferably 6.0 μm or less and further preferably 5.0 μm or less. Amicroporous layer having a surface roughness of 7.0 μm or less, morepreferably 6.0 μm or less, further preferably 5.0 μm or less is lesslikely to damage an electrolyte membrane to improve the durability of afuel cell. The gas diffusion electrode substrate having a surfaceroughness as described above can be obtained by the production methoddescribed below.

In the gas diffusion electrode substrate of the present invention, thesize (peak size) of a pore having a pore size in the range of 0.03 to1.00 μm and a maximum volume is preferably in the range of 0.10 to 1.00μm and more preferably in the range of 0.10 to 0.80 μm. A peak size inthe range of 0.10 to 1.00 μm, more preferably in the range of 0.10 to0.80 μm can more effectively suppress the flooding.

In the still other aspect of the gas diffusion electrode substrate ofthe present invention, the gas diffusion electrode substrate has avariety in thickness of 10.0 μm or less. The gas diffusion electrodesubstrate has a variety in thickness of more preferably 9.0 μm or lessand further preferably 8.0 μm or less. A gas diffusion electrodesubstrate having a variety in thickness of 10.0 μm or less, morepreferably 9.0 μm or less, further preferably 8.0 μm or less reduces thesurface roughness of the gas diffusion electrode substrate and is lesslikely to damage an electrolyte membrane to improve the durability of afuel cell. Here, the variety in thickness of the gas diffusion electrodesubstrate can be determined using a micrometer in a state of compressingthe gas diffusion electrode substrate at a surface pressure of 0.15 MPa.The gas diffusion electrode substrate having a variety in thickness asdescribed above can be obtained by the production method describedbelow. Although the variety in thickness does not particularly have alower limit, it is normally 1 μm or more.

The gas diffusion electrode substrate having a peak size in the range of0.10 to 1.00 μm can be obtained by controlling the primary particlessize and the state of dispersion of the carbon powder.

Here, a pore size distribution of the gas diffusion electrode substrate(distribution of the volume of a pore with respect to the size of thepore) has been obtained by mercury intrusion technique. Three testpieces each in a rectangle of about 12 mm×20 mm have been cut out fromthe gas diffusion electrode substrate, precisely weighed, and placed ina measurement cell so as not to be piled, and mercury has been injectedunder reduced pressure. Then, the measurement has been conducted underthe following conditions.

Measurement pressure range: Pressure at start of measurement 6 kPa (poresize 400 μm) to pressure at end of measurement 414 MPa (pore size 30 nm)

Measurement cell mode: Pressurization process in the above pressurerange

Cell volume: 5 cm³

Surface tension of mercury: 485 dyn/cm

Contact angle of mercury: 130°

As the measurement apparatus, AutoPore 9520 manufactured by ShimadzuCorporation or its equivalent product can be used.

The size (peak size) of a pore having a pore size in the range of 0.03to 1.00 μm and a maximum volume has been also determined from the poresize distribution.

A method for producing a gas diffusion electrode substrate according tothe present invention is a method for producing a gas diffusionelectrode substrate including a carbon sheet and a microporous layer,the carbon sheet being porous, the method including an application stepof applying a coating liquid for forming the microporous layer onto atleast one surface of the carbon sheet by a slit die coater, the slit diecoater having a lip-tip length of 0.10 to 10.00 mm, and the MPL coatingliquid containing a carbon powder that has a DBP oil absorption of 70 to155 ml/100 g and an ash content of less than 0.10% by mass, andcontaining a dispersion medium.

Hereinafter, the method for producing a gas diffusion electrodesubstrate according to the present invention is specifically describedwith reference to an example of using, as the carbon sheet, a carbonfiber baked substrate obtained from a carbon fiber papermakingsubstrate.

<Papermaking Substrate and Production Method of Papermaking Substrate>

In order to obtain a carbon fiber-containing papermaking substrate,employed is, for example, a wet papermaking method in which a carbonfiber-containing papermaking substrate is produced by dispersing carbonfibers in a liquid, a dry papermaking method in which a carbonfiber-containing papermaking substrate is produced by dispersing carbonfibers in the air. Especially, a wet papermaking method is preferablyemployed because of its excellent productivity.

For the purpose of improving the water removal performance and the gasdiffusivity in the in-plane direction of the carbon sheet, carbon fiberscan be mixed with an organic fiber to make paper. As the organic fiber,there can be used a polyethylene fiber, a vinylon fiber, a polyacetalfiber, a polyester fiber, a polyamide fiber, a rayon fiber, an acetatefiber, or the like.

Further, for the purpose of improving the shape-retaining property andease of handling of the papermaking substrate, an organic polymer can beincorporated as a binder. Here, as the organic polymer, there can beused polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, cellulose,or the like.

For the purpose of maintaining the in-plane electrical conductivity andthermal conductivity to be isotropic, the papermaking substrate ispreferably in the form of a sheet in which carbon fibers are randomlydispersed in a two-dimensional plane.

Although the pore size distribution obtained for the papermakingsubstrate is influenced by the content rate and the dispersion state ofcarbon fibers, pores can be formed generally at a size of about 20 to500 μm.

In the papermaking substrate, the areal weight of the carbon fiber ispreferably in the range of 10 to 45 g/m², more preferably in the rangeof 15 to 40 g/m², and further preferably in the range of 20 to 35 g/m².An areal weight of the carbon fiber of 10 g/m² or more, more preferably15 g/m² or more, further preferably 20 g/m² or more preferably makes thecarbon sheet excellent in mechanical strength. An areal weight of thecarbon fiber of 45 g/m² or less, more preferably 40 g/m² or less,further preferably 35 g/m² or less preferably makes the carbon sheetexcellent in gas diffusivity in the through-plane direction and waterremoval performance. In a case of laminating a plurality of papermakingsubstrates, the areal weight of the carbon fiber after the lamination ispreferably in the above ranges.

Here, the areal weight of the carbon fiber in the carbon sheet can bedetermined by retaining a papermaking substrate cut in a 10-cm square inan electric furnace under a nitrogen atmosphere at a temperature of 450°C. for 15 minutes and dividing the weight of the residue obtained byremoval of organic matters by the area of the papermaking substrate(0.01 m²).

<Impregnation with Resin Composition>

As a method for impregnating a carbon fiber-containing papermakingsubstrate with a resin composition, there can be employed a method ofdipping a papermaking substrate into a resin composition, a method ofapplying a resin composition onto a papermaking substrate, a method oflaminating and transferring a film composed of a resin composition ontoa papermaking substrate, or the like. Especially, a method of dipping apapermaking substrate into a resin composition is preferably employedbecause of its excellent productivity.

In the present invention, a preferable aspect is that the covering rateof one surface of the carbon sheet by a carbon fiber and a bindermaterial be different from the covering rate of the other surface by acarbon fiber and a binder material. This configuration can be obtainedby distributing more resin composition, which is to be the bindermaterial on the carbon sheet, to one surface of a porous material whenthe porous material is impregnated with the resin composition.

By entirely uniformly impregnating a porous material such as a carbonfiber-containing papermaking substrate with a resin composition by, forexample, dipping and then, removing an excessively loaded resincomposition from one surface of the porous material before drying, theamount of the resin composition on the front and rear surfaces of aprepreg can be controlled, so that the value of the covering rate on onesurface of a resultant carbon sheet can be controlled to be differentfrom the value of the other surface.

As one example, by dipping a carbon fiber papermaking substrate into aresin composition-containing solution, and then, drawing the resincomposition-containing solution out of one surface of the carbon fiberpapermaking substrate before drying or bringing a squeeze roll intocontact with only one surface of the carbon fiber papermaking substratebefore drying, the loading amount of the resin composition in thevicinity of one surface of the carbon fiber papermaking substrate can bereduced, compared to the loading amount of the resin composition in thevicinity of the other surface.

As another example, also by dipping a carbon fiber papermaking substrateinto a resin composition-containing solution and then, additionallyapplying a resin composition onto only one surface of the carbon fiberpapermaking substrate by spraying or a gravure roll, the value of thecovering rate on one surface of the carbon sheet can be controlled to bedifferent from the value of the other surface.

As still another example, during drying after a carbon fiber papermakingsubstrate is dipped into a resin composition-containing solution, byloading the resin composition to one surface of the carbon fiberpapermaking substrate more by gravity on the resin composition andhot-air drying from the one surface, the value of the covering rate onone surface of the carbon sheet can be controlled to be different fromthe value of the other surface.

In any of the above cases, by entirely incorporating the resincomposition into the carbon fiber papermaking substrate, an excessivedifference in the covering rate on the front and rear surfaces of thecarbon sheet can be suppressed, so that excellent entire binding isobtained and gas diffusivity becomes excellent.

The resin composition used for preparing a prepreg is preferably a resincomposition that is carbonized during baking to be an electricallyconductive carbide as a binder. Here, the resin composition refers toone obtained by adding, for example, a solvent to a resin component asrequired. The “resin component” is one that contains a resin such as athermosetting resin and, as required, an additive(s) such as a carbonpowder and a surfactant.

More in detail, the carbonization yield of the resin component containedin the resin composition is preferably 40% by mass or more. Acarbonization yield of 40% by mass or more preferably makes the carbonsheet excellent in mechanical characteristics, electrical conductivity,and thermal conductivity. Although the carbonization yield does notparticularly have an upper limit, it is normally about 60% by mass.

In the present invention, examples of the resin constituting the resincomponent in the resin composition used to prepare a prepreg includesthermosetting resins such as a phenolic resin, an epoxy resin, amelamine resin, and a furan resin. Especially, a phenolic resin ispreferably used because of its high carbonization yield. In addition,the resin composition can also be used as a binder material withoutcarbonizing the resin composition, and there can be used thermoplasticresins such as a fluororesin, an ABS resin, a polyethylene resin, anacrylic resin, a nylon resin, a polypropylene resin, a polyethyleneterephthalate resin, or a urethane resin.

As the additive(s) added, as required, as the resin component in theresin composition used to prepare a prepreg, a carbon powder can be usedfor the purpose of improving the mechanical characteristics, theelectrical conductivity, and the thermal conductivity of the carbonsheet. Here, as the carbon powder, there can be used carbon black suchas furnace black, acetylene black, lamp black, and thermal black;graphite such as flaky graphite, scaly graphite, earthy graphite,artificial graphite, expanded graphite, and flake graphite; a carbonnanotube; a carbon nanofiber; a milled fiber of a carbon fiber; or thelike.

As the resin composition used to prepare a prepreg, the resin componentsdescribed above can be used as they are and can also be incorporatedinto various solvents, as required, for the purpose of enhancing theimpregnation property of the resin composition into a porous materialsuch as a carbon fiber papermaking substrate. Here, as the solvent,there can be used methanol, ethanol, isopropyl alcohol, or the like.

The resin composition used to prepare a prepreg is preferably in aliquid form at a temperature of 25° C. and in a state of 0.1 MPa. Aliquid resin composition is excellent in the impregnation property intoa papermaking substrate to make a resultant carbon sheet excellent inmechanical characteristics, electrical conductivity, and thermalconductivity.

In the present invention, the resin component is impregnated in anamount of preferably 30 to 400 parts by mass and more preferably 50 to300 parts by mass relative to 100 parts by mass of the carbon fiber in aprepreg. An amount of the impregnated resin component of 30 parts bymass or more relative to 100 parts by mass of the carbon fiber in aprepreg makes the carbon sheet excellent in mechanical characteristics,electrical conductivity, and thermal conductivity. Meanwhile, an amountof the impregnated resin component of 400 parts by mass or less makesthe carbon sheet excellent in gas diffusivity in the in-plane directionand in the through-plane direction.

<Lamination and Annealing>

A carbon fiber-containing papermaking substrate is impregnated with aresin composition to prepare a prepreg and then, prepregs can belaminated or a prepreg can be subjected to annealing prior tocarbonization.

For the purpose of making the carbon sheet have a prescribed thickness,a plurality of prepregs can be laminated. In this case, a plurality ofprepregs having the same property can be laminated, or a plurality ofprepregs having different properties can be laminated. Specifically, itis possible to laminate a plurality of prepregs that are different interms of, for example, the mean diameter and the mean length of thecarbon fiber, the areal weight of the carbon fiber in the papermakingsubstrate, or the amount of the impregnated resin component.

For the purpose of increasing the viscosity of the resin composition orpartially cross-linking the resin composition, the prepreg can besubjected to annealing. As an annealing method, there can be employed amethod of blowing hot air against the prepreg, a method of heating theprepreg by sandwiching it between hot platens of, for example, a pressapparatus, a method of heating the prepreg by sandwiching it betweencontinuous belts, or the like.

<Carbonization>

After impregnating a carbon fiber-containing papermaking substrate withthe resin composition, the resulting papermaking substrate is baked inan inert atmosphere to perform carbonization. For this baking, abatch-type heating furnace or a continuous heating furnace can be used.The inert atmosphere can be obtained by allowing an inert gas such as anitrogen gas or an argon gas to flow in the furnace.

In the present invention, the highest temperature in the baking ispreferably in the range of 1300 to 3000° C., more preferably in therange of 1700 to 3000° C., and further preferably in the range of 1900to 3000° C. A highest temperature of 1300° C. or more proceedscarbonization of the resin component to preferably make the carbon sheetexcellent in electrical conductivity and thermal conductivity.Meanwhile, a highest temperature of 3000° C. or less preferably reducesthe operating cost of a heating furnace.

In the present invention, a “carbon fiber baked substrate” refers to oneobtained by impregnating a carbon fiber-containing papermaking substratewith the resin composition and then carbonizing the resultingpapermaking substrate.

<Hydrophobic Treatment>

For the purpose of improving the water removal performance, the carbonfiber baked substrate may be subjected to a hydrophobic treatment.

The hydrophobic treatment can be performed by applying a fluororesinonto the carbon fiber baked substrate and annealing the resulting bakedsubstrate. Here, examples of the fluororesin includepolytetrafluoroethylene (PTFE), a tetrafluoroethylene-hexa fluoropropylene copolymer (FEP), and a tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA). The application amount of the fluororesin ispreferably 1 to 50 parts by mass and more preferably 3 to 40 parts bymass relative to 100 parts by mass of the carbon fiber baked substrate.An application amount of the fluororesin of 1 part by mass or morepreferably makes the carbon sheet excellent in water removalperformance. Meanwhile, an application amount of the fluororesin of 50parts by mass or less preferably makes the carbon sheet excellent inelectrical conductivity. The carbon fiber baked substrate is, after theapplication of the fluororesin, preferably subjected to drying at 90° C.or more and less than 200° C.

The carbon fiber baked substrate corresponds to the “carbon sheet.” Asdescribed above, the carbon fiber baked substrate is subjected to ahydrophobic treatment as required, and in the present invention, thecarbon fiber baked substrate having a hydrophobic treatment performedthereon is to also correspond to the “carbon sheet” (the carbon fiberbaked substrate not having a hydrophobic treatment performed thereonnaturally corresponds to the “carbon sheet”).

<Formation of Microporous Layer>

In the method for producing a gas diffusion electrode substrateaccording to the present invention, the microporous layer can be formedby applying an MPL coating liquid containing a carbon powder and adispersion medium onto at least one surface of the carbon sheet. The MPLcoating liquid preferably contains a fluororesin.

FIG. 1 shows a schematic sectional view of a production apparatus in themethod for producing a gas diffusion electrode substrate according tothe present invention. An MPL coating liquid 1 is delivered to a slitdie coater by a metering pump and pushed out from a slit 3 of a die 2 tobe applied onto a carbon sheet 4.

The MPL coating liquid may contain a dispersant such as a surfactant. Asthe dispersion medium, for example, water or an organic solvent can beused. Especially, the MPL coating liquid preferably contains water asthe dispersion medium because rapid drying of the MPL coating liquid inan MPL application step sometimes induces microcracks on a surface ofthe microporous layer. As the dispersant, a nonionic surfactant is morepreferably used. As the carbon powder, carbon black as described aboveis preferably used; however, the MPL coating liquid may contain variouscarbon powders other than carbon black.

In the present invention, the carbon powder in the MPL coating liquidand in the microporous layer needs to have a DBP oil absorption in therange of 70 to 155 ml/100 g. The carbon powder has a DBP oil absorptionof more preferably 150 ml/100 g or less and further preferably 145ml/100 g or less. The carbon powder also has a DBP oil absorption ofmore preferably 80 ml/100 g or more and further preferably 90 ml/100 gor more. A carbon powder having a DBP oil absorption of 70 ml/100 g ormore, more preferably 80 ml/100 g or more, further preferably 90 ml/100g or more improves the dispersibility thereof to give an MPL coatingliquid high in storage stability. Further, a carbon powder having a DBPoil absorption of 70 ml/100 g or more, 80 ml/100 g or more, furtherpreferably 90 ml/100 g or more increases the viscosity of the MPLcoating liquid to suppress the infiltration of the MPL coating liquidinto the carbon sheet in the MPL application step, reducing the surfaceroughness of the microporous layer. In addition, the infiltration of theMPL coating liquid into the carbon sheet is suppressed to increase theporosity of a carbon sheet portion in the gas diffusion electrodesubstrate, improving the gas diffusivity to improve the cellperformance. A carbon powder having a DBP oil absorption of 155 ml/100 gor less, more preferably 150 ml/100 g or less, further preferably 145ml/100 g or less decreases the secondary particle size of the carbonpowder to reduce the surface roughness of the microporous layer. The DBPoil absorption can be increased by raising the impact rate of particlesof a raw material at the time of producing the carbon powder. The DBPoil absorption of the carbon powder contained in the MPL coating liquidcan be determined in accordance with JIS K 6217-4 (2008 amendedversion).

In the method for producing a gas diffusion electrode substrateaccording to the present invention, the carbon powder in the MPL coatingliquid has an ash content of necessarily less than 0.10% by mass, morepreferably less than 0.07% by mass, and further preferably less than0.02% by mass. The ash content inhibits a catalyst reaction in a fuelcell, and therefore, a carbon powder having an ash content of less than0.10% by mass, more preferably less than 0.07% by mass, furtherpreferably less than 0.02% by mass improves the durability of a fuelcell that includes the gas diffusion electrode substrate of the presentinvention. In addition, a carbon powder having an ash content of lessthan 0.10% by mass, more preferably less than 0.07% by mass, furtherpreferably less than 0.02% by mass reduces the volume specificresistance thereof to give a gas diffusion electrode substrate excellentin electrical conductivity. The ash content of the carbon powdercontained in the MPL coating liquid can be determined in accordance withJIS K 6218-2 (2005 established version).

When the carbon powder contained in the MPL coating liquid has a DBP oilabsorption of 70 to 155 m/100 g, more preferably 80 to 150 ml/100 g,further preferably 90 to 145 ml/100 g and having an ash content of lessthan 0.1% by mass, more preferably less than 0.07% by mass, furtherpreferably less than 0.02% by mass, the storage stability of the MPLcoating liquid particularly increases.

Example of the method of reducing the ash content include selection of araw material for producing the carbon powder, which contains a minimumamount of alkali metals such as sodium, potassium, and calcium, oralkaline-earth metals. Alternatively, the ash content can also bereduced by a method of washing the carbon powder with water orhydrochloric acid to remove a part of the ash content.

In the method for producing a gas diffusion electrode substrateaccording to the present invention, all the carbon powder(s) containedin 100% by mass of the MPL coating liquid is preferably in the range of10 to 50% by mass. Here, “all the carbon powder(s)” represents, in acase of using one carbon powder, the percent (%) by mass of the carbonpowder, and, in a case of using two or more carbon powders, the totalpercent (%) by mass of all the carbon powders. All the carbon powder(s)is more preferably 12% by mass or more and further preferably 14% bymass or more. All the carbon powder(s) is also preferably 45% by mass orless and further preferably 40% by mass or less. With all the carbonpowder(s) being 10% by mass or more, more preferably 12% by mass ormore, further preferably 14% by mass or more in 100% by mass of the MPLcoating liquid, the viscosity of the MPL coating liquid appropriatelyincreases to suppress the infiltration of the MPL coating liquid intothe carbon sheet, increasing the gas diffusivity, so that a resultantgas diffusion electrode substrate becomes high in cell performance Inaddition, with all the carbon powder(s) being 50% by mass or less, morepreferably 45% by mass or less, further preferably 40% by mass or lessin 100% by mass of the MPL coating liquid, the concentration of the MPLcoating liquid decreases to reduce the variety in areal weight of themicroporous layer even when the application amount of the MPL coatingliquid is varied, giving a gas diffusion electrode substrate small invariety of the thickness.

In the method for producing a gas diffusion electrode substrateaccording to the present invention, the application of the MPL coatingliquid onto at least one surface of the carbon sheet needs to beperformed using a slit die coater. The use of the slit die coater canreduce the variety in the application amount of the MPL coating liquidto reduce the variety in thickness of the gas diffusion electrodesubstrate and to reduce the surface roughness of the microporous layer.

The microporous layer is preferably formed by a plurality ofapplications of the MPL coating liquid. A plurality of applications canmore reduce the surface roughness of the microporous layer.

In the method for producing a gas diffusion electrode substrateaccording to the present invention, the slit die coater shown in FIG. 1needs to have a lip-tip length L of 0.10 to 10.00 mm, the MPL coatingliquid needs to contain a carbon powder, and the carbon powder needs tohave a DBP oil absorption of 70 to 155 m/100 g and an ash content ofless than 0.1% by mass.

The slit die coater has a lip-tip length of more preferably 0.30 mm ormore and further preferably 0.50 mm or more. The slit die coater alsohas a lip-tip length of more preferably 8.00 mm or less and furtherpreferably 6.00 mm or less. A slit die coater having a lip-tip length of0.10 mm or more, more preferably 0.30 mm or more, further preferably0.50 mm or more can form a stable liquid pool under a lip on an upstreamside to improve the application stability of the MPL coating liquid. Aslit die coater having a lip-tip length of 10.00 mm or less, morepreferably 8.00 mm or less, further preferably 6.00 mm or less isunlikely to compress the MPL coating liquid with a lip-tip portion tosuppress the infiltration of the MPL coating liquid into the carbonsheet.

Here, the lip-tip length of the slit die coater means the portionsindicated by L in FIG. 1. The slit die coater has two lip-tips in adirection in which the carbon sheet flows, and it is important in thepresent invention that at least one of the two lip-tips has a length Lof 0.10 to 10.00 mm. That is, as long as the slit die coater has atleast one lip-tip having a length L of 0.10 to 10.00 mm, it does notmatter if the other lip-tip either has a length L of 0.10 to 10.00 mm oris out of this range.

In the method for producing a gas diffusion electrode substrateaccording to the present invention, it is preferred that after theapplication of the MPL coating liquid onto the carbon sheet in the MPLapplication step, the MPL coating liquid-applied carbon sheet beretained horizontally for 1 second or more and 5 minutes or less, dried,and sintered, that is, after the application of the MPL coating liquidonto the carbon sheet, the carbon coating liquid-applied gas diffusionelectrode substrate be retained horizontally for 1 second or more and 5minutes or less before drying. That is, such a gas diffusion electrodesubstrate is retained so as to be horizontal. Here, the term“horizontal” means a state of a plane that can be kept horizontal to anextent not to allow the MPL coating liquid in the gas diffusionelectrode substrate to move in the plane. Horizontal retention for 1second or more levels the MPL coating liquid to reduce the surfaceroughness of a resultant gas diffusion electrode substrate. A retentiontime of 5 minutes or less suppresses the infiltration of the MPL coatingliquid into the carbon sheet to give a gas diffusion electrode substratehigh in gas diffusivity. A retention time of 5 minutes or less alsoimproves the productivity of the gas diffusion electrode substrate.

In the method for producing a gas diffusion electrode substrateaccording to the present invention, the MPL coating liquid has, at ashear rate of 17 s⁻¹ a viscosity preferably in the range of 1.0 to 20.0Pa·s, more preferably in the range of 2.0 to 17.0 Pa·s, and furtherpreferably in the range of 3.0 to 15.0 Pa·s. The viscosity is measuredby a shear rate and shear stress control type viscometer. Bytemperature-adjusted viscometer so that the MPL coating liquid isadjusted to 23° C., the viscosity at a shear rate of 17 s⁻¹ is measuredusing a cone with a cone angle of 1 degree. As the shear rate and shearstress control type viscometer, the shear rate and shear, stress controltype rheometer RC30 manufacture by VISCOTECH CO., LTD. or its equivalentproduct can be used.

In the method for producing a gas diffusion electrode substrateaccording to the present invention, it is preferred that one surface ofthe carbon sheet have a covering rate of 70 to 90%, the other surfacehave a covering rate lower than the covering rate of the one surface by5 to 20 points, and the MPL coating liquid be applied onto the onesurface of the carbon sheet (surface having a larger covering rate) inthe MPL application step.

With the carbon sheet having a covering rate of 70% or more on a surfaceonto which the MPL coating liquid is applied, the MPL coating liquid isless likely to infiltrate in the MPL application step to reduce thesurface roughness of the microporous layer. Meanwhile, with the carbonsheet having a covering rate of 90% or less on a surface onto which theMPL coating liquid is applied, the gas penetrant diffusivity in thethrough-plane direction of the carbon sheet is further improved to moreincrease the gas diffusivity in the through-plane direction of aresultant gas diffusion electrode substrate, improving the cellperformance at both high and low temperatures. In consideration of thebalance between efficient water removal performance and efficient gasdiffusivity, the carbon sheet preferably has a covering rate of 75.0% ormore and 81.4% or less on the one surface.

The MPL coating liquid is preferably dried at a temperature of 80 to150° C. after the application of the coating liquid onto the carbonsheet, to remove a dispersion medium. That is, the applied matter isplaced in a dryer whose temperature is set at 80 to 150° C. and dried inthe range of 5 to 30 minutes. The drying air volume may be properlydecided, but rapid drying is not desirable because microcracks aresometimes induced on the surface of the microporous layer. As describedabove, solid contents in the MPL coating liquid (carbon powder,fluororesin, surfactant, etc.) are remained after the drying.

The dried applied matter is placed in a muffle furnace, a baking furnaceor a high-temperature drying machine and heated at 300 to 380° C. for 1to 20 minutes, preferably 5 to 20 minutes to melt the fluororesin, andsintering is performed with the melted fluororesin as a binder ofparticles of the carbon powder.

In a case of using a tetrafluoroethylene-hexa fluoro propylene copolymer(FEP) as the fluororesin in either the MPL application step or thehydrophobic treatment step, the temperature of a muffle furnace, abaking furnace or a. high-temperature drying machine is preferably 370°C. or less. With the temperature set at 370° C. or less, the pyrolysisof the tetrafluoroethylene-hexa fluoro propylene copolymer (FEP) can besuppressed.

The microporous layer produced by the method for producing a gasdiffusion electrode substrate according to the present inventionpreferably has a surface roughness of 3.0 to 7.0 μm. The microporouslayer has a surface roughness of more preferably 6.0 μm or less andfurther preferably 5.0 μm or less. A microporous layer having a surfaceroughness of 7.0μm or less, more preferably 6.0, further preferably 5.0μm is less likely to damage an electrolyte membrane to improve thedurability of the electrolyte membrane.

<Catalyst Application Step>

In the present invention, a gas diffusion electrode refers to one thatis obtained by forming a catalyst layer on a surface of the microporouslayer of the gas diffusion electrode substrate.

In a case of using the gas diffusion electrode substrate of the presentinvention for a fuel cell, a catalyst coating liquid is preferablyapplied onto a surface of the microporous layer. The catalyst coatingliquid is applied onto the microporous layer, so that the adhesion ishigh between the microporous layer and the catalyst layer to increasethe electrical conductivity.

The catalyst coating liquid preferably contains a solid polymerelectrolyte, a carbon-supported catalyst, and a dispersion medium. As acatalyst, platinum is normally used. In a fuel cell in which a carbonmonoxide-containing reformed gas is supplied to an anode side, platinumand ruthenium are preferably used as catalysts of the anode side. As thesolid polymer electrolyte, a perfluorosulfonic acid-based polymermaterial is preferably used that has high protonic conductivity, highoxidation resistance, and high heat resistance.

<Membrane Electrode Assembled Body>

The gas diffusion electrode is joined with at least one surface of asolid polymer electrolyte membrane, with a catalyst layer side of thegas diffusion electrode directed to the solid polymer electrolytemembrane side, to constitute a membrane electrode assembled body.

<Fuel Cell>

A fuel cell of the present invention includes a bipolar plate on bothsides of the membrane assembled body. That is, the membrane electrodeassembled body and the bipolar plate on both sides of the assembled bodyconstitute the fuel cell. Normally, a plurality of built-up bodies arestacked, each of which is obtained by sandwiching such a membraneelectrode assembled body between bipolar plates on both sides, with agasket interposed between the membrane electrode assembled body and eachof the bipolar plates, to constitute a polymer electrolyte fuel cell.Such a fuel cell unit and the configuration of a fuel cell themselvesare well known.

EXAMPLES

Hereinafter, the present invention is specifically described by way ofexamples. Described below are materials, a carbon sheet, a gas diffusionelectrode substrate, a method for preparing a gas diffusion electrode,and a battery performance evaluation method of a fuel cell that are usedin the examples.

<Preparation of Carbon Sheet>

The polyacrylonitrile-based carbon fiber “TORAYCA” (registeredtrademark) T300 (mean carbon fiber diameter: 7 μm) manufactured by TorayIndustries, Inc. were cut at a mean length of 12 mm and dispersed inwater to continuously make paper by, a wet papermaking method. Further,a 10% by mass aqueous solution of polyvinyl alcohol was applied onto thepaper as a binder and dried to prepare a papermaking substrate having acarbon fiber areal weight of 30 g/m². The application amount ofpolyvinyl alcohol was 22 parts by mass relative to 100 parts by mass ofthe papermaking substrate.

Using a resin obtained by mixing a resol type phenolic resin and anovolak type phenolic resin at 1:1 by mass ratio as a thermosettingresin, flaky graphite (mean particle size 5 μm) as a carbon powder, andmethanol as a solvent, the materials were mixed at an addition ratio ofthermosetting resin/carbon powder/solvent=10 parts by mass/5 parts bymass/85 parts by mass, and the resulting mixture was stirred for 1minute using an ultrasonic dispersion apparatus to give a uniformlydispersed resin composition.

A papermaking substrate cut in 15 cm×12.5 cm was dipped into the resincomposition filling an aluminum tray and then squeezed by sandwichingthe papermaking substrate between horizontally disposed two rolls. Atthis time, the loading amount of resin components on the carbon fiberpapermaking substrate was adjusted by changing the clearance between thehorizontally disposed two rolls. After the impregnation, the papermakingsubstrate was heated for drying at 100° C. for 5 minutes to prepare aprepreg. Next, the prepreg was annealed at 180° C. for 5 minutes whilebeing pressurized by a pressing machine with flat plates. At the time ofthe pressurization, a spacer was disposed in the pressing machine toadjust the gap between the upper and lower counter plates.

A substrate obtained by annealing the prepreg was introduced into aheating furnace maintaining a nitrogen gas atmosphere and having ahighest temperature of 2400° C., to give a carbon sheet formed of acarbon fiber baked substrate.

PTFE was imparted in an amount of 5 parts by mass relative to 95 partsby mass of the carbon fiber baked substrate, and the PTFE-imparted bakedsubstrate was heated for drying at 100° C. for 5 minutes to prepare acarbon sheet with a thickness of 150 μm and an areal weight of 46 g/m².

<Formation of Microporous Layer>

Using a slit die coater having a lip-tip length shown in tables, an MPLcoating liquid was applied onto a surface of the carbon sheet to form amicroporous layer. The used slit die coater had lip-tip lengths that arethe same on upstream and downstream sides. The MPL coating liquid usedhere was obtained by using acetylene black as a carbon powder, which hada DBP oil absorption and an ash content that are shown in the tables,PTFE (“POLYFLON” (registered trademark) D-1E manufactured by DaikinIndustries, Ltd.) as fluororesin, a surfactant (“TRITON” (registeredtrademark) X-100 manufactured by Nacalai Tesque, Inc.), and purifiedwater as a dispersion medium, and by adjusting the addition amounts ofthe materials so that the MPL coating liquid would have compositionshown in the tables that indicate the addition amounts by part by mass.The addition amount of PTFE shown in the tables represents an additionamount of a water dispersion liquid of PTFE. The MPL coating liquid wasapplied onto the carbon sheet and then heated (sintered) at 100° C. for10 minutes and at 380° C. for 10 minutes to prepare a gas diffusionelectrode substrate. The delivery amount of the MPL coating liquid wasadjusted by a metering pump so that the microporous layer would have anareal weight of 18 g/m².

<Formation of Catalyst Layer>

Using a slit die coater, a catalyst layer was formed on a surface of themicroporous layer of the gas diffusion electrode substrate. A catalystcoating liquid used here was prepared by sequentially adding 1.00 g of acarbon-supported platinum (manufactured by Tanaka Kikinzoku Kogyo K. K.,platinum supporting amount: 50% by mass), 1.00 g of purified water, 8.00g of a “NAFION” (registered trademark) solution (manufactured bySigma-Aldrich Co. LLC., “NAFION” (registered trademark), 5.0% by mass),and 18.00 g of isopropyl alcohol (manufactured by Nacalai Tesque, Inc.),and by dispersing the materials by a disperser. The catalyst coatingliquid was applied onto the surface of the microporous layer using aslit die coater and then heated at 100° C. for 10 minutes to prepare agas diffusion electrode. The amount applied by the slit die coater wasadjusted so that the catalyst layer would have an areal weight of 5g/m².

<Evaluation on Cell Performance of Polymer Electrolyte Fuel Cell>

The solid polymer electrolyte membrane “NAFION” (registered trademark)NRE-211CS (manufactured by E. I. du Pont de Nemours and Company) cut in10 cm×10 cm was sandwiched between two gas diffusion electrodes each cutin 5 cm×5 cm, with a catalyst layer side of each of the gas diffusionelectrodes directed to the solid polymer electrolyte membrane, and waspressed by a pressing machine with flat plates at a pressure of 3 MPaand at 130° C. for 5 minutes to prepare a membrane electrode assembledbody.

The resultant membrane electrode assembled body was incorporated into aunit cell for fuel cell evaluation, and the voltage was measured whenthe current density was changed. Here, used as a bipolar plate was aserpentine-type bipolar plate having one flow channel of 1.0 mm inchannel width, 1.0 mm in channel depth, and 1.0 mm in rib width. Theevaluation was carried out with non-pressurized hydrogen andnon-pressurized air being supplied town anode side and a cathode side,respectively. The evaluation was carried out while hydrogen and air wereboth humidified by a humidification pot to keep the relative humidity at100%. The utilization ratios of hydrogen and atmospheric oxygen were setat 70 mol % and 40 mol %, respectively. The output voltage was measuredat a setting of an operation temperature of 40° C., a relative humidityof 100%, and a current density of 1.5 A/cm², and the measured value wasused as an index of cell performance.

<Measurement of Areal Weight (W) of Microporous Layer>

The areal weights of the carbon sheet and the gas diffusion electrodesubstrate were determined by dividing the mass of a sample cut in a10-cm square by the area of the sample (0.01 m²) The difference in arealweight between the gas diffusion electrode substrate and the carbonsheet was defined as the areal weight (W) of the microporous layer.

<Measurement of Variety in Thickness (L) of Microporous Layer and GasDiffusion Electrode Substrate>

The carbon sheet and the gas diffusion electrode substrate were eachplaced on a smooth surface plate, and the difference in height wasmeasured at a pressure of 0.15 MPa between when the object to bemeasured was present and when the object to be measured was not present.Different 10 parts were sampled, and the average of the measured valuesof difference in height was defined as a thickness. The difference inthickness between the gas diffusion electrode substrate and the carbonsheet was defined as the thickness (L) of the microporous layer.

The variety in thickness of the gas diffusion electrode substrate wasdetermined by calculating the standard deviation in thickness of the 10parts.

<Measurement of Surface Roughness>

The surface roughness of the microporous layer was determined by depthdistribution measurement for the surface of the microporous layer usinga surface analysis laser microscope. Used as an apparatus was the lasermicroscope VK-X100 manufactured by KEYENCE CORPORATION with an objectivelens having a magnification of 10 times.

First, the gas diffusion electrode substrate was fixed onto a surfaceplate, with the microporous layer kept upward, not floated, andcreaseless, and then, a surface in the range of 5 sq. mm of themicroporous layer was measured for the depth distribution using thelaser microscope. The arithmetic average roughness was determined fromdata obtained by subjecting the depth distribution to plane automaticinclination correction. The measurement was performed for randomlyselected 10 points on the surface of the microporous layer, and theaverage value of arithmetic average roughness at the 10 points wasdefined as the surface roughness.

Example 1

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 1. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 1. The flooding resistance was also excellent asdescribed in Table 1.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Carbon sheet Covering rate [%] of one surface 74 74 74 74 74 74 Coveringrate [%] of other surface 74 74 74 74 74 74 Difference in covering rate[point] 0 0 0 0 0 0 Composition of MPL Carbon powder [part by mass] 11.011.0 11.0 11.0 11.0 11.0 coating solution Fluororesin [part by mass] 6.06.0 6.0 6.0 6.0 6.0 Surfactant [part by mass] 22.0 22.0 22.0 22.0 22.022.0 Purified water [part by mass] 61.0 61.0 61.0 61.0 61.0 61.0Production method of Application method Slit die Slit die Slit die Slitdie Slit die Slit die gas diffusion Lip-tip length [mm] 5.00 5.00 5.005.00 5.00 5.00 electrode substrate Viscosity of MPL coating solution [Pa· s] 0.9 1.5 2.5 3.5 1.9 2.9 MPL coating solution application surfaceOne surface One surface One One One One surface surface surface surfaceRetention time [s] 350 2 4 350 2 350 Gas diffusion Ash content in carbonpowder [% by mass] 0.01 0.01 0.01 0.01 0.08 0.05 electrode substrate DBPoil absorption of carbon powder [ml/100 g] 75 85 147 152 130 130 Arealweight (W) of microporous layer [g/m²] 18 18 18 18 18 18 Thickness (L)of microporous layer [μm] 21 22 23 21 22 23 Infiltration index (L/W)1.17 1.22 1.28 1.17 1.22 1.28 Variety in thickness [μm] 9.8 9.4 8.4 9.89.5 9.8 Surface roughness [μm] 6.8 6.6 6.5 6.8 6.7 6.8 Peak size of pore[μm] 0.08 0.09 0.18 0.19 0.16 0.15 Flooding resistance Output voltage[V] 0.33 0.36 0.38 0.33 0.36 0.33

Example 2

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 1. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 1. The flooding resistance was also excellent asdescribed in Table 1.

Example 3

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 1. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 1. The flooding resistance was remarkablyexcellent as described in Table 1.

Example 4

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 1. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 1. The flooding resistance was also excellent asdescribed in Table 1.

Example 5

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 1. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 1. The flooding resistance was also excellent asdescribed in Table 1.

Example 6

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 1. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 1. The flooding resistance was also excellent asdescribed in Table 1.

Example 7

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 2. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 2. The flooding resistance was remarkablyexcellent as described in Table 2.

TABLE 2 Example Comparative Example 7 Example 8 Example 9 10 Example 11Example 1 Carbon sheet Covering rate [%] of one surface 74 79 74 79 7974 Covering rate [%] of other surface 74 69 74 69 69 74 Difference incovering rate [point] 0 10 0 10 10 0 Composition of MPL Carbon powder[part by mass] 11.0 11.0 11.0 11.0 11.0 11.0 coating solutionFluororesin [part by mass] 6.0 6.0 6.0 6.0 6.0 6.0 Surfactant [part bymass] 22.0 22.0 22.0 22.0 22.0 22.0 Purified water [part by mass] 61.061.0 61.0 61.0 61.0 61.0 Production method of Application method Slitdie Slit die Slit die Slit die Slit die Slit die gas diffusion Lip-tiplength [mm] 0.20 0.40 9.00 7.00 4.00 4.00 electrode substrate Viscosityof MPL coating solution [Pa · s] 9.0 9.0 9.0 9.0 9.0 25.0 MPL coatingsolution application surface One One One One One surface One surfacesurface surface surface surface Retention time [s] 70 10 4 10 10 10 Gasdiffusion Ash content in carbon powder [% by mass] 0.01 0.01 0.01 0.010.01 0.01 electrode substrate DBP oil absorption of carbon powder[ml/100 g] 130 130 130 130 130 160 Areal weight (W) of microporous layer[g/m²] 18 18 18 18 18 18 Thickness (L) of microporous layer [μm] 25 2523 25 30 25 Infiltration index (L/W) 1.39 1.39 1.28 1.39 1.67 1.39Variety in thickness [μm] 8.5 5.9 8.6 6.1 5.8 10.1 Surface roughness[μm] 6.5 5.5 6.5 5.5 4.0 8.0 Peak size of pore [μm] 0.15 0.16 0.16 0.150.16 0.21 Flooding resistance Output voltage [V] 0.38 0.42 0.38 0.420.45 —

Example 8

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 2. At this time, used as one of twosqueeze rolls was a smooth metal roll having a structure capable ofremoving an excessive resin with a doctor blade, and as the other rollwas a roll configured to be a so-called gravure roll having projectionsand recesses. The carbon fiber papermaking substrate was sandwichedbetween the metal roll on one surface side and the gravure roll on theother surface side to squeeze the impregnation liquid of the resincomposition, differentiating the loading amount of the resin componentson the one surface from the other surface of the carbon fiberpapermaking substrate. The doctor blade was attached to the squeeze rollin contact with the other surface of the carbon sheet to remove theresin composition more from the other surface, so that a carbon sheetwas obtained that had a difference in the covering rate between the onesurface and the other surface of the carbon sheet. Further, the catalystcoating liquid was applied according to the method described in<Formation of Catalyst Layer> to give a gas diffusion electrode. As aresult of evaluating the gas diffusion electrode substrate, the surfaceroughness was remarkably excellent as described in Table 2. The floodingresistance was also remarkably excellent as described in Table 2.

Example 9

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 2. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was excellentas described in Table 2. The flooding resistance was also remarkablyexcellent as described in Table 2.

Example 10

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 2. At this time, used as one of twosqueeze rolls was a smooth metal roll having a structure capable ofremoving an excessive resin with a doctor blade, and as the other rollwas a roll configured to be a so-called gravure roll having projectionsand recesses. The carbon fiber papermaking substrate was sandwichedbetween the metal roll on one surface side and the gravure roll on theother surface side to squeeze the impregnation liquid of the resincomposition, differentiating the loading amount of the resin componentson the one surface from the other surface of the carbon fiberpapermaking substrate. The doctor blade was attached to the squeeze rollin contact with the other surface of the carbon sheet to remove theresin composition more from the other surface, so that a carbon sheetwas obtained that had a difference in the covering rate between the onesurface and the other surface of the carbon sheet. Further, the catalystcoating liquid was applied according to the method described in<Formation of Catalyst Layer> to give a gas diffusion electrode. As aresult of evaluating the gas diffusion electrode substrate, the surfaceroughness was remarkably excellent as described in Table 2. The floodingresistance was also remarkably excellent as described in Table 2.

Example 11

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 2. At this time, used as one of twosqueeze rolls was a smooth metal roll having a structure capable ofremoving an excessive resin with a doctor blade, and as the other rollwas a roll configured to be a so-called gravure roll having projectionsand recesses. The carbon fiber papermaking substrate was sandwichedbetween the metal roll on one surface side and the gravure roll on theother surface side to squeeze the impregnation liquid of the resincomposition, differentiating the loading amount of the resin componentson the one surface from the other surface of the carbon fiberpapermaking substrate. The doctor blade was attached to the squeeze rollin contact with the other surface of the carbon sheet to remove theresin composition more from the other surface, so that a carbon sheetwas obtained that had a difference in the covering rate between the onesurface and the other surface of the carbon sheet. Further, the catalystcoating liquid was applied according to the method described in<Formation of Catalyst Layer> to give a gas diffusion electrode. As aresult of evaluating the gas diffusion electrode substrate, the surfaceroughness was remarkably excellent as described in Table 2. The floodingresistance was also remarkably excellent as described in Table 2.

Comparative Example 1

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 2. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was large andinsufficient as described in Table 2. The flooding resistance was alsoinsufficient as described in Table 2, so that electric power could notbe generated.

Comparative Example 2

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 3. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was remarkablylarge and insufficient as described in Table 3. The flooding resistancewas also insufficient as described in Table 3, so that electric powercould not be generated.

TABLE 3 Comparative Comparative Comparative Comparative ComparativeComparative Example 2 Example 3 Example 4 Example 5 Example 6 Example 7Carbon sheet Covering rate [%] of one surface 74 74 74 74 74 74 Coveringrate [%] of other surface 74 74 74 74 74 74 Difference in covering rate[point] 0 0 0 0 0 0 Composition of Carbon powder [part by mass] 11.011.0 11.0 11.0 11.0 11.0 MPL coating Fluororesin [part by mass] 6.0 6.06.0 6.0 6.0 6.0 solution Surfactant [part by mass] 22.0 22.0 22.0 22.022.0 22.0 Purified water [part by mass] 61.0 61.0 61.0 61.0 61.0 61.0Production method Application method Slit die Slit die Gravure SpraySlit die Slit die of gas diffusion Lip-tip length [mm] 4.00 4.00 — —12.00 0.05 electrode substrate Viscosity of MPL coating solution [Pa ·s] 0.1 0.1 9.0 9.0 9.0 9.0 MPL coating solution application surface Onesurface One surface One surface One surface One surface One surfaceRetention time [s] 10 10 10 10 10 10 Gas diffusion Ash content in carbon0.01 0.12 0.01 0.01 0.01 0.01 electrode substrate powder [% by mass] DBPoil absorption of carbon 65 130 130 130 130 130 powder [ml/100 g] Arealweight (W) of microporous 18 18 18 18 18 18 layer [g/m²] Thickness (L)of microporous 5 5 9 5 5 — layer [μm] Infiltration index (L/W) 0.28 0.280.50 0.28 0.28 — Variety in thickness [μm] 12 12 13.5 16 12 — Surfaceroughness [μm] 13.0 13.0 8.0 14.0 13.0 — Peak size of pore [μm] 0.070.15 0.16 0.15 0.16 — Flooding resistance Output voltage [V] — — — — — —

Comparative Example 3

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 3. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was remarkablylarge and insufficient as described in Table 3. The flooding resistancewas also insufficient as described in Table 3, so that electric powercould not be generated.

Comparative Example 4

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 3, except that the application method waschanged to gravure printing. Further, the catalyst coating liquid wasapplied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was remarkablylarge and insufficient as described in Table 3. The flooding resistancewas also insufficient as described in Table 3, so that electric powercould not be generated.

Comparative Example 5

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 3, except that the application method waschanged to spray painting. Further, the catalyst coating liquid wasapplied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was remarkablylarge and insufficient as described in Table 3. The flooding resistancewas also insufficient as described in Table 3, so that electric powercould not be generated.

Comparative Example 6

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 3. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. As a result of evaluating thegas diffusion electrode substrate, the surface roughness was remarkablylarge and insufficient as described in Table 3. The flooding resistancewas also insufficient as described in Table 3, so that electric powercould not be generated.

Comparative Example 7

A gas diffusion electrode substrate was obtained according to themethods described in <Preparation of Carbon Sheet> and <Formation ofMicroporous Layer> above and by applying an MPL coating liquid havingthe composition shown in Table 3. Further, the catalyst coating liquidwas applied according to the method described in <Formation of CatalystLayer> to give a gas diffusion electrode. The gas diffusion electrodesubstrate had loading unevenness of the microporous layer, and theflooding resistance was insufficient as described in Table 3, so thatelectric power could not be generated.

DESCRIPTION OF REFERENCE SIGNS

1: MPL coating liquid

2: Die

3: Slit

4: Carbon sheet

L: Lip-tip length

1. A gas diffusion electrode substrate comprising a carbon sheet and amicroporous layer, wherein the carbon sheet is porous, a carbon powderincluded in the microporous layer has a DBP oil absorption of 70 to 155ml/100 g, the microporous layer has an infiltration index (L/W) of 1.10to 8.00, the infiltration index being calculated from an areal weight(W) of the microporous layer and a thickness (L) of the microporouslayer (L), and the microporous layer has a thickness (L) of 10 to 100μm.
 2. The gas diffusion electrode substrate according to claim 1,wherein the microporous layer has a surface roughness of 3.0 to 7.0 μm.3. The gas diffusion electrode substrate according to claim 1, having avariety in thickness of 10.0 μm or less.
 4. A gas diffusion electrodesubstrate comprising a carbon sheet and a microporous layer, wherein thecarbon sheet is porous, a carbon powder included in the microporouslayer has a DBP oil absorption of 70 to 155 ml/100 g, and themicroporous layer has a surface roughness of 3.0 to 7.0 μm.
 5. A gasdiffusion electrode substrate comprising a carbon sheet and amicroporous layer, wherein the carbon sheet is porous, a carbon powderincluded in the microporous layer has a DBP oil absorption of 70 to 155ml/100 g, and the gas diffusion electrode substrate has a variety inthickness of 10.0 μm or less.
 6. The gas diffusion electrode substrateaccording to claim 1, wherein the microporous layer includes as a carbonpowder a linear carbon having an aspect ratio of 30 to
 5000. 7. The gasdiffusion electrode substrate according to claim 1, wherein a size (peaksize) of a pore having a pore size in the range of 0.03 to 1.00 μm and amaximum volume is in the range of 0.10 to 1.00 μm.
 8. The gas diffusionelectrode substrate according to claim 1, wherein one surface of thecarbon sheet has a covering rate of 70 to 90%, the other surface has acovering rate lower than the covering rate of the one surface by 5 to 20points, and the microporous layer is on a side of the one surface of thecarbon sheet.
 9. A method for producing a gas diffusion electrodesubstrate including a carbon sheet and a microporous layer, wherein thecarbon sheet is porous, the method comprises an application step ofapplying a coating liquid for forming the microporous layer(hereinafter, described as an MPL coating liquid) onto at least onesurface of the carbon sheet by a slit die coater (hereinafter, describedas an MPL application step), the slit die coater has a lip-tip length of0.10 to 10.00 mm, and the MPL coating liquid contains a carbon powderhaving a DBP oil absorption of 70 to 155 ml/100 g and an ash content ofless than 0.10% by mass, and contains a dispersion medium.
 10. Themethod for producing a gas diffusion electrode substrate according toclaim 9, wherein the MPL coating liquid has a viscosity of 1.0 to 20.0Pa·s at a shear rate of 17 s⁻¹.
 11. The method for producing a gasdiffusion electrode substrate according to claim 9, wherein after theapplication of the MPL coating liquid in the MPL application step, theMPL coating liquid-applied carbon sheet is retained horizontally for 1second or more and 5 minutes or less, and subsequently, the carbon sheetis dried and sintered.
 12. The method for producing a gas diffusionelectrode substrate according to claim 9, comprising water as thedispersion medium.
 13. The method for producing a gas diffusionelectrode substrate according to claim 9, wherein all carbon powder(s)contained in 100% by mass of the MPL coating liquid is 10 to 50% bymass.
 14. The method for producing a gas diffusion electrode substrateaccording to claim 9, wherein one surface of the carbon sheet has acovering rate of 70 to 90%, the other surface has a covering rate lowerthan the covering rate of the one surface by 5 to 20 points, and the MPLcoating liquid is applied onto the one surface of the carbon sheet inthe MPL application step.
 15. A fuel cell comprising the gas diffusionelectrode substrate according to claim 1 or a gas diffusion electrodesubstrate obtained by the method according to any of claims 9 to 14.